Do naked ballots mean trouble for John Fetterman? Here’s what you need to know.

Naked ballots didn’t cost Joe Biden the 2020 election. That doesn’t mean Democrats are in the clear this year.

A down-to-the-wire U.S. Senate race in Pennsylvania has renewed some attention — albeit less than two years ago — on how mail ballots can be rejected in the state if voters make mistakes filling them out and sending them back. In addition to “naked ballots” tossed for missing required inner secrecy envelopes, voters’ ballots can be thrown out for missing the deadline, forgetting to sign them, and other errors.

The difference between votes cast and votes actually counted is usually pretty small, but some are rejected every election. Those rejections go largely unnoticed — until they matter in extremely close races.

Races like ones in Pennsylvania.

Donald Trump won the state in 2016 by less than 1% of the vote; Biden won it by just over 1%. We’ve had automatic recounts in the last two elections (this year’s Republican Senate primary and a 2021 judicial race). The high-stakes Senate race between Lt. Gov. John Fetterman, the Democratic nominee, and celebrity doctor Mehmet Oz, the Republican, could be headed for a razor-thin finish. The state is also home to some of the country’s most competitive U.S. House races .

And Democrats stand to have thousands or even tens of thousands more ballots rejected than Republicans. Here’s why:

And while rejection rates were much lower than expected in 2020, thanks partly to celebrity-studded public education campaigns for a presidential race, it’s unclear whether that will hold true this year.

» READ MORE: Everything you need to know about Pa.’s November 2022 election

Mollie Binotto, senior adviser for Pennsylvania Democrats’ statewide election efforts, said voters’ increased familiarity with mail ballots should help reduce rejections this year. The party has a digital advertising campaign, with some outreach events, that she said builds on work from 2020 “to make sure that votes are counted and that everyone has the ability to vote.”

Mail ballots are a secure way to vote; they can also be more complicated than doing it in person.

Mail voting has long had higher failure rates partly because there are more steps required and it’s harder for voters to catch mistakes.

“If it’s in person and there’s a problem with your registration, you know instantly and can start to deal with it instantly, right in front of you,” said Charles Stewart, an MIT professor who studies election administration. “Whereas if you’re voting by mail, you won’t know instantly.”

Some voters forget to sign the outside mailing envelope or they make an identifying mark on the inner secrecy envelope. Naked ballots — those returned in only the mailing envelope, without the blank inner secrecy envelope — got a huge amount of attention in 2020 because they sometimes make up a significant share of rejections.

» READ MORE: What to do if you mess up your Pennsylvania mail ballot

Court fights have changed the requirements over time. Just last week the state Supreme Court told counties to segregate and not count both undated and wrongly dated ballots. Undated ballots are those that voters return without a handwritten date on the outer envelope; the court did not immediately define what counts as an incorrectly dated ballot.

And one of the biggest reasons ballots get tossed is because they arrive after the deadline of 8 p.m. on election day. Voters can request ballots until one week before that, leaving a tight window to receive, fill out, and return them.

Experience matters. Voters who are most familiar with the electoral system — such as so-called super-voters who participate in both high- and low-turnout elections — tend to know how to successfully vote.

Some of the same factors also apply to mail ballots, said Marc Meredith, a political science professor at the University of Pennsylvania who has studied the state’s mail ballot rejections.

When he studied Philadelphia’s naked-ballot data from 2020, Meredith found that people who were new to mail voting were more likely to cast naked ballots. Hispanic and Asian voters appeared more likely than white voters to submit naked ballots, and people in neighborhoods with higher shares of college degrees were less likely to do so. Rejection patterns appeared similar in this year’s primary, Meredith said.

Naked ballots were different from other rejections in one way: age. Older voters were more likely than younger voters to cast naked ballots. But overall, younger voters will likely still have higher rejection rates, Meredith said, because ballots arriving after the deadline make up so many of the rejections.

“Part of the reason is — we’re seeing this definitely play out this election — that younger voters often wait longer to fill out their ballot than older voters,” Meredith said.

Democrats are far more likely than Republicans to vote by mail.

Trump’s attacks on the method in 2020 — based on lies about widespread fraud — helped drive Republicans away from mail ballots. And mail voters appear to be even more heavily Democratic now.

The result is that as mail ballots are rejected, that will have a greater impact on Democrats than Republicans.

“The fact that you have so many more mail ballots in Democratic hands than Republican hands, any sort of category of rejection out there is going to take away Democratic votes relative to Republican votes,” Meredith said.

Counties take different approaches to minimize rejections.

Some help voters “cure” flaws, such as by letting them sign an unsigned ballot. Republicans sued to stop those curing procedures, but they’re allowed this election after the Pennsylvania Supreme Court deadlocked on the issue.

There are things individual voters can do if they make a mistake with their ballot, but systemic processes and procedures have greater impact.

Counties have also tried other changes. Some place staffers near drop boxes to help double-check that voters filled out their envelopes correctly. Philly added large arrows to the signature and date boxes, and made the inner secrecy envelope harder to miss.

“We made our secrecy envelope a different color — they’re blue,” said Omar Sabir, one of the three city commissioners, Philadelphia’s elections board. “We don’t want any naked ballots, we want you to put blue jeans on your ballot.”

Data on ballot rejections can be messy, partly because counties may have different procedures or change them between elections. Litigation also changes which ballots can be counted.

The court order that undated and wrongly dated ballots be set aside, for example, will mean thousands of votes are rejected instead of counted — unless future litigation changes that. A more precise estimate isn’t possible right now, since counties haven’t tracked wrongly dated ballots in the past.

Generally speaking, roughly 1% to 2% of mail ballots being rejected is normal. Hit 3% and something has gone particularly wrong; get below 1% and things are going quite well.

Where exactly this election lands depends on a number of factors. Voters are more familiar with mail ballots now than in 2020. Changes counties have made may help reduce error.

On the other hand, millions of dollars were spent in 2020 on awareness campaigns and outreach, including one popular video in which celebrities including Chris Rock and Amy Schumer appeared naked to warn voters about naked ballots.

There’s less attention this year, and that could drive up the number of rejections, said Meredith, the Penn professor.

“People have gained more experience with mail balloting … so I think that will work in the favor of reducing rejected ballots,” Meredith said. “That’s going to conflict with the fact that there just isn’t this broader awareness of the importance of naked ballots, dating your affidavit, getting your ballot back on time. So if I had to predict, I think the broader environment is going to outweigh the more experience — we’re going to see more rejected ballots this time.”

Stewart, the MIT professor, isn’t so sure. Mail voting has become institutionalized, better systems including ballot curing are in place, and rejection rates usually go down in midterm elections, he said.

“I’m kind of sanguine about mail ballots this time,” Stewart said.

In Philly, Sabir urged all voters to get their ballots in as soon as possible — and to do it right: “Just remember to take your time when you read your ballot. We have instructions in your vote-by-mail ballot. And just return those ballots, we need you to return them.”